I believe that one of the more prominent Salty Dogs perpetuately laments that rust never sleeps! I chased the rust on a steel drift boat trailor for years. Nothing seemed to slow it down. I have to admit, it was on the coast and dunked in salt a lot!

I've got a dory trailer that was made out of carbon steel, and I had it acid dipped and then galvanized at a place under the Freemont bridge in Portland. That was in 1990 and it is still rust free except where the sand has scraped the zinc off. It may be spendy, but so is a new trailer. I dont remember the name of the place but I bet a web search would find a myriad of places that do this kind of work.
Doc.

I bought a can of that rubber coating stuff that is used as a bed liner at Wal-Mart...comes in an aerosol can. I sprayed it all over the fenders and tongue of my trailer about 3 years ago, haven't seen a bit of rust since.